datacenter config: add options to control tag usage rights

datacenter config: add options to control tag usage rights

By adding a 'user-tag-privileges' and 'registered-tags' option.

The former sets the general policy by which "normal" users (with just
'VM.Config.Options' on the respective guest) can set or delete tags
on a guest, and the latter is a list of registered tags only settable
by users with admin-like permission ('Sys.Modify' on '/'); it is
designed to be used as source/target for actions like backup jobs in
the future.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
 [T: drop outdated stuff & reword/work commit message]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>